Failed to open /etc/webmin/webmin/oscache for writing : Disk quota exceededI just can't do anything, everywhere this error is coming but form ssh I can see that there is lots of space available on my server. Please help me what to do here ..

There is a difference between disk quotas and having disk space. I have never used quotas before but from what I have read, it limits space based on a specific user or perhaps software. Somewhere, there is a config file that tells how much space a user can use and you need to increase that. Have a look at these two links:

It appears there is more than one software package that does this. They also seem to have different commands. You may have to find out what software you have installed to manage this before you can find out what commands to use.